benchmark: reduce code duplication

Merge duplicate case code in child-process-params.js.

PR-URL: https://github.com/nodejs/node/pull/36568


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>